The targeting construct is designed to insert a loxP- and FRT-flanked neomycin resistance cassette, in reverse orientation, downstream of exon 4. The E57K point mutation (GAG to AAG) was introduced in exon 4 resulting in a missense mutation. E57K is found in patients with autosomal recessive cutis laxa (ARCL) 1B. Flp-mediated recombination removed the FRT-flanked neo cassette.